The term airfoil, as used hereinafter, designates any body whose shape causes it to receive a useful reaction from an air stream moving relative to it. The term is usually associated with the profile or chordwise section of the wing. Such aircraft components as wings, movable control members, and stabilizers, by way of example only, are deemed to be airfoils as the term is used herein. Planform, as used hereinafter, refers to the contour of an aerodynamic surface, such as, for example, a wing, control surface or stabilizer, as viewed perpendicular to the plane of the surface. Lifting surface, as used hereinafter, refers to such aircratt components as, for example, wings, control surfaces, canards and side force generators.
Aircraft capable of extended range while cruising a supersonic Mach number typically employ highly-swept, slender wings. These delta-wings are of interest because they have the potential to obtain relatively low drag at supersonic lifting conditions. However, a familiar characteristic of these wings is the sudden formation of rather stable leading-edge vortices at off-design conditions. These leading-edge vortices have a profound influence on the wing pressure distribution and, therefore, on the aerodynamic performance, stability characteristics, and the structural design loads.
The formation of the leading-edge vorticity results because the boundary layer on the lower surface of the wing flow outward and separates as it goes over the leading edge, forming a free shear layer. The shear layer curves upward and outboard, eventually rolling up into a core of high vorticity. There is an appreciable axial component of motion and the fluid spirals around and along the axis. A spanwise outflow is induced on the upper surface, beneath the coiled vortex sheet, and the flow separates again as it approaches the leading edge, forming secondary vortex.
The size and the strength of the coiled vortex sheets increase with increasing incidence and they become a dominant feature of the flow, which remains steady throughout the range of practical flight attitudes of the wing. The formation of these vortices is responsible for the nonlinear aerodynamic characteristics that exist over a given angle-of-attack range which give rise to vortex lift.
Essentially, vortex lift control involves stabilizing the vortex shed from the leading edge of the wing so as to lock the leading edge vorticity along the spanwise direction of the wing. This causes lift-producing, streamline airflow to pass over the upper surface of the wing, over the locked vortex, and then to become reattached to the wing surface. This results in an effective increase of the wing camber and thus increased lift.
It is known that vortex stability is highly dependent on the velocity of axial flow external to the vortex core as well as the vortex swirl velocity. In the past, either sweep, such as found with a highly swept delta-wing, or some other mechanism for inducing axial flow, has been employed to organize and stabilize the leading edge vortex so as to delay the bursting thereof.
Thus, passive vortex lift control is a means to control, without adding energy, vortex burst, i.e. the point at which the vortex becomes unstable due to reaching certain critical limits of its characteristics. At the location of vortex burst, the wing lift is severely reduced. Therefore, means of controlling the vortex stability is required to delay vortex burst and increase the wing lift.
One prior art passive vortex lift control method, for inducing the necessary axial flow required to organize the separated air flow from the leading edge of the airfoil into a vortex, is the leading edge extention or strake. Typically, the strake is positioned inboard of and upstream to the airfoil and is provided with a leading edge having sufficient sharpness such that air flow separation is fixed at the leading edge thereof. The strake thus forms a stable strake vortex at a given angle of attack which induces spanwise flow from the inboard portion of the wing towards the wing tip. This axial flow, as noted above, serves to organize the air flow separation at the leading edge of the wing into a vortex.
Because almost all present day, high performance wings have some degree of aft sweep, the above-noted passive method for inducing axial flow is operationaly quite efficient. However, the modern trend in high performance aircraft is to utilize a swept forward wing configuration for which strake induced axial flow is inapplicable.